To increase printed image density in ink jet printing, so-called composite printing method is used where at least one color ink is ejected to a part of a recording medium to be printed in black in the same printing scan as that of a black ink so as to at least a part of the color image overlaps with the black image on the recording medium to form an image called “composite black.”
A drawback with the composite black is that image density is sometimes rather reduced due to the black ink seemingly overlaid with color ink. To solve this problem, the present inventors have provided a method of using a combination of a black ink comprising a copolymer of vinylpyrrolidone and a C10-14 alkene as a pigment dispersant and a cyan ink comprising a nitrogen-containing polymer grafted with polyester side chains (See Japanese Patent Laid-Open No. 2009-270061).